U.S. Pat. No. 4,026,680 addresses the problem of cold and hot spots or zones which are formed in the pressure vessels of PSA plants, particularly large scale PSA plants for the production of oxygen from feed air where the adsorbent material used is a zeolite molecular sieve.
U.S. Pat. No. 4,026,680 discloses that temperature drops in the order of 100.degree. below feed air temperature have been observed at the inlet end of pressure vessels forming part of a PSA plant for the production of oxygen from feed air. Since the molecular sieve material is usually selected to operate best, as regards oxygen recovery efficiency, at ambient conditions, the effect of hot and cold spots is to reduce the performance of the molecular sieve material.
In order to meet this problem U.S. Pat. No. 4,026,680 teaches the transference of heat within the pressure vessel from the hot spot or zone to the cold spot or zone by metal solid conduction. The metal solid conduction can take the form of a plurality of metal plates positioned parallel and equi-distant from each other and extending through the adsorbent bed. An alternative embodiment teaches the use of metal rods.
Such methods form a passive control system in as much as a temperature difference has to exist within the pressure vessel before they can operate. That is to say, they reduce rather than eliminate the hot and cold spot effect. Although the temperature differences are very pronounced, in the order of minus 70.degree. C. in the cold part of the pressure vessel and plus 40.degree. C. in the warm part of the pressure vessel, the net heat flow is relatively small and the effect of metal solid conduction can take several days to become established.